But they do see / or : in file names, and the interesting question is which one it is today on which filesystem.

The filesystems of macOS are particularly opinionated, much more than most Unices which tend toward "anything is allowed [and usually preserved] except \0 and /".

macOS supports case-insensitivity[0] and performs unicode normalization[1] on filenames, and decomposes name data to an extent that the question "what does the fs see" is a bit moot.

With that said, the internal storage of filenames in APFS are a nul-terminated UTF-8 string[2], with (i'm pretty sure) colons as colons, which the Finder displays as slashes.

[0] if you make a file named "Makefile" then touch a file named "makefile", it'll touch the first file, instead of making a second file.

[1] if you make a file named "schön" (s-c-h-combining¨-o-n) and then search for (s-c-h-ö-n), you can find it, or vice versa. The particular normalization/canonicalization used is NFD.

[2] j_drec_key_t description in https://developer.apple.com/support/downloads/Apple-File-Sys...